Dr. Ding is a member of a shrinking club of people who are pretty sure they have never been infected with SARS-CoV-2, the virus that causes Covid-19. Geneticists and immunologists are studying factors that might protect people from infection, and learning why some are predisposed to more severe Covid-19 disease.

For many, the explanation is likely that they have in fact been infected with the virus at some point without realizing it, said Susan Kline, professor of medicine at the University of Minnesota Medical School. About 40% of confirmed Covid-19 cases are asymptomatic, according to a meta-analysis published in December in the Journal of the American Medical Association…

Immunologists are studying whether exposure to one type of pathogen can trigger an immune response against others including SARS-CoV-2. A study published in the journal Science Immunology this month found some people had T-cells in their guts and on their skin before the pandemic began that appeared to help them fight SARS-CoV-2.

There is some evidence that some people who may have been exposed to certain coronaviruses before the pandemic are equipped with cells that attack SARS-CoV-2 before it can spread, said Steve Jameson at the University of Minnesota Medical School.